1. Field of the Invention
The present invention relates to a PZT piezoelectric material, a method for producing the PZT piezoelectric material, as well as a piezoelectric device and a liquid discharge device employing the PZT piezoelectric material.
2. Description of the Related Art
Piezoelectric devices, which include a piezoelectric material that expands or contracts when the intensity of an applied electric field is increased or decreased, and an electrode for applying the electric field to the piezoelectric material, are used in applications, such as piezoelectric actuators provided in inkjet recording heads. As piezoelectric materials, PZT (lead zirconium titanate) and substitution systems of PZT, which has a part of the A-site and/or B-site thereof being substituted with a different element, have been known. PZT and the substitution systems thereof are herein collectively referred to as the “PZT system” or “PZT”.
It has been known that PZT doped with a donor ion which has a higher valence than a valence of a substituted ion has higher piezoelectric performance than that of the intrinsic PZT. Examples of a donor ion that substitutes Zr4+ and/or Ti4+ at the B-site include V5+, Nb5+, Ta5+, Sb5+, Mo6+ and W6+. A PZT perovskite oxide with a part of the B-site thereof substituted with a different element M is represented by general formula (P) below:Pba(Zrx,Tiy,Mb-x-y)bOc  (P)(wherein M represents one or two or more B-site elements; wherein 0<x<b, 0<y<b, 0≦b-x-y; and wherein a molar ratio a:b:c is 1:1:3 as a standard; however, the molar ratio may be varied from the standard molar ratio within a range where a perovskite structure is obtained.)
In applications such as inkjet recording heads, it is preferred that the piezoelectric material has a high piezoelectric constant, and a high piezoelectric performance of, for example, a piezoelectric constant d31≧150 pm/V is required. Further, for the inkjet recording heads, it is preferred that the heads need to be replaced less often, and thus sufficient durability of the piezoelectric material for practical use is required.
In the PZT perovskite oxides, if a Pb content is low, the pyrochlore phase forms and this degrade the piezoelectric performance. It is therefore considered to be preferable to provide an A-site-rich composition, which has the Pb (i.e., an A-site element) content higher than a stoichiometric proportion. However, a higher Pb content tends to result in degradation of insulation resistance and durability.
A PZT dielectric body having high electric permittivity for use in a multilayer capacitor is disclosed in H. Kanai et al., “Effect of Stoichiometry on the Dielectric Properties and Life Performance of (Pb0.875Ba0.125) [(Mg1/3Nb2/3)0.5(Zn1/3Nb2/3)0.3 Ti0.2]O3 Relaxor Dielectric Ceramic Part I, Dielectric Properties”, J. Am. Ceram. Soc., Vol. 76, No. 2, pp. 454-458, 1993, and H. Kanai et al., “Effect of Stoichiometry on the Dielectric Properties and Life Performance of (Pb0.875Ba0.125) [(Mg1/3Nb2/3)0.5(Zn1/3Nb2/3)0.3 Ti0.2]O3 Relaxor Dielectric Ceramic Part II, Life Performance”, J. Am. Ceram. Soc., Vol. 76, No. 2, pp. 459-464, 1993. It is taught in these documents that an insufficient Pb content results in formation of the pyrochlore phase; however, the insulation resistance and the durability are improved, and both the high durability and high capacitance can be obtained. In the application as the dielectric body (where only high capacitance is required), the presence of the pyrochlore phase in some degree does not influence the performance of the dielectric body in use, and thus the presence of pyrochlore phase is not really a problem.
In the application as the piezoelectric material, however, even a trace of pyrochlore phase results in significant degradation of the piezoelectric performance. Therefore, a piezoelectric film which has a low Pb content and thus contains the pyrochlore phase is not applicable to practical use in the piezoelectric application.
U.S. Pat. No. 7,348,715 discloses (in claim 1) that, in the PZT system, improved durability can be provided when a molar ratio a/b between the A-site element and the B-site element is not less than 0.85 and less than 1.0. This document does not mention the pyrochlore phase. In this document, the film is formed through a usual gas phase film formation process (specifically, RF magnetron sputtering), and no measure is taken to prevent the formation of the pyrochlore phase. Thus, under the conditions described in the above document, where the Pb content is low, it is impossible to avoid the formation of the pyrochlore phase.
As described above, with respect to the PZT piezoelectric bodies, the piezoelectric performance and the durability are trade-off, and it has been impossible to provide both the high piezoelectric performance and high durability.